Giant amethyst geodes likely grew from groundwater

Amethyst geodes—some stretching 5 m long—dot the ancient lava flows of Uruguay’s Los Catalanes mining area. The mineral agate lines cavities in basalt. Colorless quartz grows atop the agate, while large violet crystals of amethyst pierce the center of the geode, which also contains calcite crystals. Researchers have debated whether the gem-studded geodes formed from hot magmatic fluids or colder liquids seeping into the rock. A new analysis of trapped fluids and oxygen isotopes in the minerals suggests that the gems grew from groundwater and crystallized at low temperatures between 15 and 60 °C ( Miner. Deposita 2024, DOI: 10.1007/s00126-024-01310 -2 ). The oxygen isotope analysis “is state of the art,” says Jens Götze, a mineralogist at Freiberg University of Mining and Technology who was not part of the work. The study demonstrates how modern analytical tools can help to solve questions about how mineral deposits have formed, he says.